The present invention relates to a pole of a circuit breaker for high- and/or medium-voltage transmission and/or distribution grids, i.e. for voltages greater than 1000 Volt, which comprises a current measuring sensor which is integrated in its structure and is realized by means of optical technologies. The pole according to the present invention is now described with reference to a pole of a high-voltage circuit breaker without thereby limiting in any way the scope of its application.
It is known that current measurements are usually performed in a pole of a high-voltage circuit breaker in order to ensure adequate control of said circuit breaker. Current measurements are generally performed by using measurement poles which are known in the art as current transformers. These measurement poles generally comprise windings on a core made of magnetic material and supporting and insulation structures. Said current measurement poles can be of various kinds and are used according to particular configurations which are described hereinafter.
A first configuration of current transformers is the one known in the state of the art as stand-alone transformer.
FIG. 1 schematically illustrates an example of a current transformer which is generally used in said configuration.
The transformer is mainly constituted by three structural components: an insulator 1, generally constituted by a finned tube made of polymeric material or porcelain; a head 2, made of aluminum or steel; and a base 3 which is also made of aluminum or steel and constitutes the structure for anchoring to a supporting surface, for example a supporting pillar.
The primary winding 5 of the transformer is positioned inside the head 2, as shown in FIG. 1, and is constituted by a through bar 6 which is arranged horizontally and fixed to the head 2 in a suitable manner.
The secondary windings 8 of the transformer are arranged inside some toroidal shields 7 and are supported by a supporting tube 9 which is fixed by means of its lower end to the base 3 of the transformer. Inside the tube 9, conductors 10 from the secondary windings 8 are conveyed and connected, at their terminals, to a terminal box 11 which is arranged at the base 3 of the transformer. A flange 12 between the base 3 and the insulator 1 has holes 13 which are required for the passage of the conductors 10 and for introducing the dielectric gas that arrives from a filling valve (not shown in the figure;) provided in the base 3. The dielectric gas can be constituted, for example, by sulfur hexafluoride (SF6), nitrogen or a mixture of the two gases.
The above described current transformer has several problems due to the use of a transformer having a magnetic core.
Under high currents the magnetic core of the transformer is in fact affected by saturation effects which compromise the current measurement to be performed. These effects force to model the transformer core according to the intensity of the currents to be measured and to the precision with which the measurement is to performed. This entails considerable engineering problems and high manufacturing costs.
Further disadvantages arise from the fact that windings with a magnetic core generally have a limited frequency band and are potentially sensitive to external electromagnetic interference.
These disadvantages lead to high production and operating costs which increase as the operating voltages rise, due to the need to use high-quality magnetic cores in order to ensure adequate repeatability of the performance of the measurement pole.
The stand-alone transformer configuration has, as described hereinafter, considerable problems in terms of bulk and high costs both during installation and during operation.
FIG. 2 is a schematic view of an example of use of said stand-alone transformer configuration in a high-voltage substation in which the pole shown in FIG. 1 can be used as a current transformer.
The line current flows, for example in the direction of the arrow 24, across a disconnector 20 to a circuit breaker 1 and from there to a current transformer 22, already described in FIG. 1. Access to the remaining part of the substation is gained by means of the disconnector 23.
The current transformer 22 can be arranged both upstream and downstream of the circuit breaker 21 but in any case it is arranged outside the circuit breaker 21. In order to ensure adequate insulation for each electrical pole of the line the transformer 22 must be placed on a separate support and located at a suitable distance from the circuit breaker 21. This entails a considerable overall space occupation of the substation. This fact leads to high installation and operating costs. The plurality of different and separate functional elements inside the substation furthermore entails considerable problems in terms of maintenance and reliability.
FIG. 3 is a schematic view of an example of configuration in which integration between the circuit breaker and the current transformer is provided in a single pole. In particular, as described in FIG. 3, said integration is performed inside the body of the circuit breaker. The circuit breaker/current transformer assembly is mainly constituted by three parts, respectively an interruption chamber 30, shown partially in FIG. 3, a region 31 which accommodates primary windings and secondary windings 34 of the transformer (provided on a magnetic core), an insulator 33 and a housing 32 which accommodates means 35 for the actuation of the moving contact of the circuit breaker and secondary terminals 36 of the transformer. Conductors 37 which protrude from the windings 34 are conveyed through a metal tube 38 located inside the insulator 33 to the secondary terminals 36. Said metal tube 38 also accommodates a rod 39 for actuating a moving contact 40 of the circuit breaker. The primary current flows from the moving contact 40 to an external primary contact 41 which is located at the region 31 that accommodates the windings 34.
Although the pole of FIG. 3 advantageously mutually integrates the current measurement pole and the circuit breaker, it still uses current transformers wound on a magnetic core. In this configuration as in others which can be found in the art, the technological problems arising from the use of these components therefore remain. As described earlier, said technological problems are essentially the large space occupation and high costs of the windings and the non-ideal magnetic behavior of the core of these transformers.
There are other known poles which allow to solve the problems that arise from the use of windings on magnetic cores. These poles use optical technologies and are based on the measurement of the rotation of the polarization plane of a light wave which propagates through a transmission medium in the presence of a magnetic field. The rotation is proportional to the intensity of the magnetic field. This property is commonly known as Faraday effect. For the sake of descriptive simplicity, poles of this type are termed hereinafter xe2x80x9coptical current sensorsxe2x80x9d. FIG. 4 schematically illustrates a first known constructive example of optical current sensor.
An optical fiber 53 is wound on a suitable support (not shown in the figure) around a primary conductor 51 through which there flows a current (represented by the arrow 52) to be measured. A control system 54 sends a light wave (represented by the arrow 55) which travels along the optical fiber 53. Along its path, the light wave 55 emitted by the control system 54 is influenced by the magnetic field (represented by the dashed arrow 50) generated by the current 52. Said light wave 55 returns to the control system 54 with its polarization angle rotated by a certain extent. The control system 54 measures this rotation. As already noted the extent of this rotation is proportional to the magnetic field 50 and therefore to the current 52 that flows along the primary conductor. The sensitivity of the optical sensor according to this embodiment depends essentially on the number of turns of the optical fiber 53 around the primary conductor. The behavior of the sensor is independent of the geometry of the turns of said optical fiber.
Another known constructive example of optical current sensor is presented in FIG. 5. According to this embodiment, the transmission medium used is a crystal 56 having a suitable geometry and arranged so as to encircle the primary conductor 51 like a ring. A control system 57 emits a light wave (represented by the arrow 58) which, by means of a system of optical fibers 59, reaches the crystal 56. Inside the crystal 56, the light wave 58 undergoes a series of reflections which make said light wave 58 travel along a path around the primary conductor 51 until it returns through the optical fiber system 59 to the control block 57, which measures the rotation of the polarization plane of the light wave 58. This rotation is due to the presence of the magnetic field 50 generated by the current 52 to be measured. As in the example of FIG. 4, the extent of said rotation is proportional to the intensity of the magnetic field 50 and therefore to the intensity of the current 52.
With respect to the preceding embodiment, this one is less used because the sensitivity of the sensor can be increased only by increasing the number of internal reflections of the light wave 58 and therefore the dimensions of the crystal 56. This fact can cause, beyond a certain limit, considerable problems in manufacturing said crystal 58. The embodiment of FIG. 5 is therefore used for measuring relatively high currents, for example above 2000 amperes.
There are many known configurations alternative to the ones shown in FIGS. 4 and 5 for optical current sensors.
There are, for example, known embodiments which use multiple light waves which propagate along the same fiber or crystal in opposite directions. These embodiments are particularly advantageous in that they considerably improve the precision and sensitivity of the measurement system.
Optical current sensors generally have a high linearity even for very wide current ranges. Accordingly, they allow to advantageously solve the saturation problems that are characteristic of current transformers which have windings on a magnetic core. Furthermore, the use of optical materials such as fibers or crystals allows to ensure adequate insulation while maintaining compact dimensions.
Another advantage is constituted by the fact that optical sensors, especially those that use an optical fiber as a transmission medium for the light wave, can have highly variable geometries while maintaining their functionality unchanged.
Another advantage is constituted by the fact that with optical current sensors it is possible to measure AC and DC currents with the same pole.
Furthermore, the considerable development of technologies for manufacturing optical fibers and crystals allows a high degree of industrial repeatability of these sensors at competitive manufacturing prices.
As in the case of current transformers which comprise windings on a magnetic core, optical current sensors are used to measure currents in electric power transmission and/or distribution systems.
FIG. 6 schematically illustrates a constructive example of a high-voltage pole which uses optical sensors to measure the line current. As shown, a circuit breaker 60 is mounted on a support 61. The line current (represented by the arrow 62) flows in a suitable conductor 64 from the circuit breaker 60 toward a measurement probe 63 which is mounted externally with respect to the structure of the circuit breaker 60 and is insulated from it by means of a retention ring 65 which prevents the escape of the dielectric gas (for example SF6) from the circuit breaker 60.
The conductor 64 passes through a region in which the optical current sensor 66 is placed so as to be crossed by the conductor 64 in the manner described above in FIGS. 4 and 5.
Two optical fibers 67 protrude from the optical sensor 66 and are connected to a control block 68 which contains light emitting means 69, light receiving means 70, and a processing block 71. Said processing block 71, in addition to controlling the light emitting means 69 and the light receiving means 70, measures the rotation of the polarization plane of the light transmitted along the fibers 67 by the magnetic field generated by the current 62.
Alternative configurations exist for the use of optical current sensors in poles of high-voltage circuit breakers. However, in the current state of the art said optical sensors are always placed in structures located externally with respect to the circuit breaker, as shown in FIG. 6, or in configurations of the stand-alone transformer type, similar to the one shown in FIG. 7.
With reference to said FIG. 7, an optical sensor 730 is mounted externally with respect to the body of a circuit breaker 740 at one of the main current conductors (designated by the arrow 750).
A cable 760 descends along an insulator 770 which is required to provide the adequate spacing between the current conductor 750 and the ground plane which is rigidly coupled to the processing electronics 780. The insulator 770 is also used to protect the optical cable 760 against the effects of weather. As an alternative, the current sensor can be arranged at the other current conductor 790.
One disadvantage of these embodiments is the need to use, for current measurement, elements which are external to the structure of the circuit breaker. This entails the need to use external supports and protections (for example supporting columns and/or insulators), consequently increasing the dimensions and therefore the manufacture and installation costs, as described above with respect to the embodiment of FIG. 2.
Furthermore, the use of several structural elements increases maintenance problems and decreases the reliability of the system.
The aim of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker which comprises a pole for measuring current which allows to perform very accurate measurements and allows a considerable reduction in space occupation.
Within the scope of this aim, an object of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker which comprises a current measurement pole in which current measurement occurs without having the non-ideal conditions typical of current measurements performed by means of windings on a core of magnetic material.
Another object is to provide a pole of a high- and/or medium-voltage circuit breaker which comprises a current measurement pole in which the insertion of said pole entails a reduced number of components required for the practical execution of the structure of said circuit breaker.
Another object of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker in which the insertion of a current measurement pole entails a reduced number of mechanical processes to be performed in order to produce the structure in practice.
Another object of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker in which the insertion of a current measurement pole entails a reduced number of electrical connections to be performed for the operation of said pole.
Another object of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker in which the step for the assembly of said current measurement pole can be performed simply and quickly.
Another object of the present invention is to provide a pole of a high- and/or medium-voltage circuit breaker which is highly reliable and at competitive costs. This aim, these objects and others which will become apparent hereinafter are achieved by a pole of a high- and/or medium-voltage circuit breaker, comprising an insulating housing, at least one interruption chamber which is positioned inside the insulating housing and contains at least a moving contact and at least a fixed contact, a device for measuring the electric current flowing through the pole, and a dielectric gas, characterized in that said device for measuring the electric current flowing through the pole comprises an optical current sensor arranged within a volume of the pole that is occupied by the dielectric gas.
Preferably, the optical current sensor is placed inside the structure of the interruption chamber of the circuit breaker.
In the pole according to the invention, the use of an optical current sensor ensures that the measurement of the electric current occurs accurately, without having the non-ideal conditions typical of known poles which use windings on a magnetic core.
Thanks to the insertion of the optical current sensor in the volume of the pole that contains the dielectric gas, and particularly inside the structure of the interruption chamber, the pole according to the invention allows to considerably reduce space occupation, using a reduced number of components and electrical connections required for its operation.
The pole according to the invention is furthermore very easy to assemble, does not require particular mechanical processes for its manufacture, is highly reliable and has relatively low manufacturing costs.